BHODemon is a free tool that will list all Browser Helper Objects that are installed on a Windows system by scanning the registry and give you the ability to disable them. This will also list "good" BHOs as well, but nevertheless is a useful tool in detecting and disabling malicious software.

On June 24th, a visitor to the SANS Internet Storm Center reported that his company was "...in the middle of a very disturbing ... issue regarding the adware/spyware/IE exploit genre..." He requested help analyzing an "encrypted or compressed" file that had been downloaded to a machine at their site. Tom Liston, one of our volunteer handlers, spent the weekend analyzing this issue. His findings are summarized here.

The victim of the attack found that a file called "img1big.gif" had been loaded onto their machine. Because of the account restrictions on the person running the machine, it had failed to install properly, which was why it had come to their attention. It is this file that they forwarded to the SANS Internet Storm Center for analysis.

The file is not a graphic file at all. It is actually a 27648 byte Win32
executable that has been compressed using the Open Source executable compressor UPX. This file decompresses to an 81920 byte file which contains two Win32 executables bound together. The first portion of the file (and what actually runs if the file extension is changed and the program is launched) is a "file dropper" Trojan, designed to install any executable concatenated to its body. The second half of the file consists of a Win32 DLL that is installed by the file dropper under WindowsXP as a randomly named .dll file under C:\WINDOWS\System32\. This DLL is installed as a "Browser Helper Object" (BHO) under Internet Explorer.

A "Browser Helper Object" is a DLL that allows developers to customize and control Internet Explorer. When IE 4.x and higher starts, it reads the registry to locate installed BHO's and then loads them into the memory space for IE. Created BHO's then have access to all the events and properties of that browsing session. This particular BHO watches for HTTPS (secure) access to URLs of several dozen banking and financial sites in multiple countries.

When an outbound HTTPS connection is made to such a URL, the BHO then grabs any
outbound POST/GET data from within IE before it is encrypted by SSL. When it captures data, it creates an outbound HTTP connection tohttp://www.refestltd.com/cgi-bin/yes.pl
and feeds the captured data to the script found at that location.